As society has generally been developed, the quantity of water used has increased, and a sewage treatment apparatus has been used for treating the used water. In such a sewage treatment apparatus, a biological treatment process has been frequently used, and a physical-chemical treatment process, in addition to the biological treatment process, has been recently used.
This biological treatment process uses microorganisms to dissolve pollutant materials, and waste sludge may be generated when microorganisms are used to dissolve pollutant materials. Further, in the physical-chemical treatment process, chemicals are added and mixers or the like are used to mix the chemicals, so that pollutant materials can be quickly and efficiently removed. However, the physical-chemical treatment process is uneconomical because the continuous supply of chemicals, the increase of the generated sludge, the stirring operation, and the like result in the increase of treatment costs. Further, the physical-chemical treatment process is undesirable in that if the amount of chemicals is too small to remove pollutant materials, the removal efficiency of pollutant materials may be reduced, while if the amount of chemicals is too large, other pollutions may be generated due to the misappropriation of chemicals.
Meanwhile, although the biological treatment process may not quickly remove pollutant materials, there is an advantage in that the biological treatment process is stable, reliable and economical in a long-term view and does not generate another environmental pollution. Nevertheless, the biological treatment process leaves much room for improvement so far.
The conventional sewage treatment system receives water so that a state of the sewage treatment system may be changed into one of an anaerobic state, an anoxic state and an aerobic state, and the sewage treatment system includes a sewage treatment apparatus, which has a biological reactor whose a discharge portion is controlled to be opened/closed in order to change a flow path, and a sewage treatment control device for controlling the sewage treatment apparatus according to the set values.
The sewage treatment apparatus which is controlled by various processes as described above, controls the sewage treatment processes under different conditions depending on the water quality of the inflow sewage, so that the sewage treatment performance and the economic efficiency can be improved and another environmental pollution can be minimized.
The sewage treatment control device as described above has used a temporal control method in which on/off operation is repeated during a predetermined interval by means of PLC, DCS and PC control devices, a quantitative control method in which a constant target value is set for quantitative control, a manual control method in which a target value is manually controlled by an operator, or a programmable control method in which a given variable target value program is used for control.
When the constant target value is used to operate the sewage treatment control device, it is impossible to quickly cope with an external environmental change. Meanwhile, the manual control method in which the target value is manually controlled depends on the specialty of an operator. Accordingly, if the operator has no specialty, appropriate control is impossible and a sewage treatment process may be controlled under the subjective control condition of the operator.
Accordingly, a method for controlling a sewage treatment process has been recently used, in which a target value is appropriately set depending on a water quality load and a water quality state, and a variable target value is set by a given program.
Meanwhile, the conventional biological sewage treatment process does not effectively remove pollutant materials such as nitrogen and phosphorous in addition to BOD (Biological Oxygen Demand) and SS (concentration of suspended solids) because pollutant materials such as living wastewater, industrial wastewater and livestock wastewater have increased and secondary treatment facilities of the activated sludge method is limited, and therefore, the water pollution has been really going on.
Accordingly, apparatuses and methods for effectively removing pollutant materials such as nitrogen and phosphorous have been developed, and the methods are classified into a physical-chemical treatment method and a biological treatment method depending on their treatment manner.
The biological sewage treatment method includes an anaerobic zone, an anoxic zone, an aerobic zone, or the like, and may be represented as an A2/O method, an intermittent aeration method and an SBR method.
In the aforementioned A2/O series sewage treatment apparatus, internal partition walls are installed within a biological reactor to spatially separate the biological reactor into small chambers in order to identify an anaerobic (zone) state, an anoxic (zone) state and an aerobic (zone) state. The A2/O series sewage treatment apparatus is very useful for a large-scaled sewage treatment, is relatively resistant to load variations, and maintains a stably treated water quality which is over a predetermined level.
Referring to FIG. 1, which is a flowchart of the conventional A2/O series sewage treatment apparatus, the conventional A2/O series sewage treatment apparatus allows inflow water, such as sewage and polluted waste water, which has been settled and treated in a primary clarifier to partially remove suspended solid materials, to be introduced into a biological reactor.
The internal portion of the biological reactor is divided into an anaerobic zone 10, an anoxic zone 12 and an aerobic zone 14 by partition walls. Further, the inflow water which has passed through the biological reactor is stored in a secondary clarifier 16, in which foreign materials are settled and then the water is discharged outward.
At this time, the sludge generated in the secondary clarifier 16 is reintroduced into the anaerobic zone, so that the retreatment process is performed.
Further, in order to improve the removal efficiency of nitrogen, ammonia nitrogen should be converted into nitrate nitrogen in the aerobic (zone) state, and then, an internal recycle pump or a propeller-type submarine transfer apparatus should be used to perform the internal recycle into the anoxic (zone) state.
As described above, the A2/O series sewage treatment apparatus separates the treatment stages from each other by the partition walls, whereas the intermittent aeration and SBR series sewage treatment apparatuses separately operate the anaerobic state, the anoxic state and the aerobic state via temporal control.
That is, the conventional intermittent aeration or SBR series sewage treatment apparatuses has one biological reactor, and is controlled to operate the biological reactor in the anaerobic state, the anoxic state and the aerobic state under different conditions with the passage of time, without requiring the internal recycle as described in the conventional A2/O series sewage treatment apparatus.
As such, the conventional sewage treatment methods identify the anaerobic state, the anoxic state and the aerobic state via the spatial or temporal separation (control), and are operated while the minimum dissolve oxygen concentration of 2.0 mg/L or more is maintained in the aerobic state. In addition, a mixer is installed in the biological reactor in the anoxic or anaerobic state, so that the contents therein may be completely mixed by a physical method, and if the internal recycle is performed, an internal recycle pump or the like is required.
Meanwhile, the operational control of the sewage treatment apparatus is mainly dependent on conditions, such as the inflow amount of sewage, a pollutant load in inflow sewage, and microorganism concentration in the biological reactor, and these conditions are determined by a flow rate measuring apparatus, a water quality measuring apparatus, an experimental value, and the like.
The inflow amount of sewage, which is most influential among these operational conditions of the sewage treatment apparatus, is highly fluid depending on time, day, month and season. Accordingly, if the sewage treatment apparatus is operated by a determined quantitative control condition, much time and budget may be wasted to maintain the normal operation, and thus, it is not easy to implement the normal operation. Further, since a change of operational conditions depending on experimental values causes an experiment procedure to be complicated and to require much time, it is difficult to cope with the conditions such as the flow rate and the pollutant load amount which are changed in real time. When a target value is manually determined, there is a problem in that a permanently stationed manager should adjust the target value depending on variable conditions.
Recently, although many automatic operational control methods using flow rate sensors and water quality measuring sensors have been developed, an error range in the measuring sensitivity of the water quality measuring sensors increases and frequency of maintenance occurrence for the sensors increases with the passage of time in comparison with their initial installment, so that there may be technical problems such as component replacement and periodic maintenance and economical problems such as component replacement and dual installment cost. As such, the operational control of the sewage treatment process depending on the sensors has not been really implemented.
FIG. 2 is a block diagram schematically showing a known sewage treatment control device of a sewage treatment system. Referring to FIG. 2, a sewage treatment control device 50 is equipped with a measuring unit 51, which has sensors for inspecting specific components in water, and the measuring unit 51 is connected to a target value setting unit 52. As the component values measured from the measuring unit 51 are input, the target value setting unit 52 sets target values of operational conditions required for the sewage treatment.
In addition to the method in which the sewage treatment control device 50 is automatically operated by the set values measured by the measuring unit 51, the sewage treatment control device 50 has a manual setting unit 53 in which a target value is set depending on a component value which is input by an operator. Further, the sewage treatment control device 50 is equipped with a signal selection unit 55 for selectively connecting either the target value setting unit 52 or the manual setting unit 53, and an automatic/manual mode selection unit 54 in which an operator may select either an automatic mode or a manual mode to select the signal selection unit 55. Further, the target value input from the signal selection unit 55 is transferred to a PID control unit 56 and is converted into a control signal by proportional, integral or derivative action. In addition, the PID control unit 56 is connected to a sewage treatment apparatus 57, and the sewage treatment apparatus 57 is manipulated depending on the converted control signal to treat the sewage.
Meanwhile, the sewage treatment control device 50 includes a control compensation unit in which the target value inputted from the signal selection unit 55 and the measured value measured from the water that has passed through the sewage treatment apparatus 57 are compared with each other and the target value to be inputted to the PID control unit 56 may then be compensated. The control compensation unit includes a final measuring unit 58, which has a sensor installed to an outlet side of the sewage treatment apparatus 57 to inspect a specific component of water, and a comparison unit 59, which is installed to an inlet side of the PID control unit 56 to compare the specific component value anticipated from the set target value and the specific component value inspected by the final measuring unit 58 and correct the target value so that the anticipated specific component value may be converged into the inspected specific component value.
Although the sewage treatment system configured as described above conventionally measures one specific component with one water quality measuring unit, a difference in sensitivity of the sensor may be generated as the water quality measuring unit has been operated for a long time. Accordingly, a measuring error and an economical problem such as maintenance occur.
In order to solve these problems, a scheme has been proposed in the conventional sewage treatment system, in which a plurality of water quality measuring units are installed, so that the measuring values between the water quality measuring units are compared with each other, or when there is a failure of one water quality measuring unit, the disabled water quality measuring unit may be replaced with another water quality measuring unit to measure the water quality. However, since the water quality measuring unit is expensive, the scheme has not been used as a realistic alternative scheme.
Accordingly, most sewage treatment systems conventionally used have not been used for a long time after the initial installation due to an increase of maintenance or incessant adjustments of the sensors. There is a problem in that it does not quickly cope with external factors such as extinction of microorganisms, a failure of the system, variations in flow rate of the inflow water and variations in the load amount, so that the function of the sewage treatment process may be lost.
Further, although the conventional sewage treatment system provides a monitoring and control system using the real-time water quality monitoring, it dose not provide a system which may check whether or not the monitoring and control system is abnormal, and therefore, the reliability for the monitoring and control system is considerably low. In addition, if the conventional sewage treatment plant is modified by an advanced treatment method, the period of time required to install partition walls and submarine machinery is so long that the treatment efficiency for the sewage treatment plant may be dramatically reduced during the construction period of time, thereby aggravating the pollutions in the neighboring rivers.
Further, in the conventional sewage treatment system, high dissolved oxygen concentration is maintained in an aerobic state and a mixer is installed and continuously operated in an anaerobic or anoxic state, large operational cost is necessary to operate the sewage treatment apparatus.
Therefore, there exists a need for the improvement of the sewage treatment apparatus in the conventional sewage treatment system as well as the fundamental improvement of the control method for controlling the sewage treatment apparatus.